Dynamic range shows the ratio of a specified maximum level of a parameter to the minimum detectable value of that parameter. High dynamic range imaging allows a greater dynamic range of exposures than normal digital imaging techniques. For example, a high dynamic range image can accurately represent the wide range of intensity levels found in real scenes, ranging from direct sunlight to the dark shadows, which may exceed the ratio of 10000 to 1.
A high dynamic range scene can be captured through a set of differently exposed photographs of the scene. Through changing the exposure duration, details in different portions of the high dynamic range can be captured in the differently exposed photographs.
For example, a typical digital camera has a limited dynamic range. With a given exposure setting, the digital camera may not be able to capture the details in a very bright area in the scene, since the bright area in the picture taken by the digital camera is saturated and represented as a nearly uniform white region. This situation becomes worse when the digital image is coded with 8 bits per channel as in JPEG or TIFF files. Similarly, the details in a very dark area in the scene may be captured as a nearly uniform black region. Different exposure durations allow the limited dynamic range of the digital camera to capture different ranges of the scene. Reducing the exposure duration can increase the captured details in the bright region; and increasing the exposure duration can increase the captured details in the dark region. When a number of images of same scene are taken with different exposure durations, the high dynamic radiance range of the scene can be recovered in a high dynamic range image.
Paul E. Debevec and Jitendar Malik (Recovering high dynamic range radiance maps from photographs, Proc. of SIGGRAPH 97, Computer Graphics Proceedings, Annual Conference Series, pp. 369-378, 1997), presents a method to derive a response curve that relates the pixel image values and the natural logarithm of the product of irradiance and exposure duration. The response curve can then be used to recover high dynamic range radiance maps from series of images of a same scene taken at different exposure levels. In one example, Paul E. Debevec and Jitendar Malik map the logarithm of the radiance values into a gray scale image, which presents all the information in the series of images taken at the different exposure levels.
Photographers who use “wet” darkroom techniques produce improved images using known procedures such as “dodging” and/or “burning” when exposing a print to a negative through a conventional photographer's enlarger. In the “wet” darkroom, some light can be withheld in a process referred to as “dodging” from a portion of the print using a small wand during exposure of the print; and more light can be added to a portion of the print using a piece of paper with a hole for “burning” during the exposure of the print. Through selectively changing the amount of light used during the development process, a photographer can selectively lighten or darken different regions of the print. Also see, “Photographic Tone Reproduction for Digital Images”, Reinhard, et al., ACM Transactions on Graphics, 21(3), pp 267-276, July 2002 (Proceedings of SIGGRAPH 2002).
However, producing a high dynamic range image for display or printing often requires an output device, such as a CRT or LCD display or a color printer, to also be able to provide a high dynamic range output. Unfortunately, many existing displays or printers do not provide a high dynamic range output.